In optical communications networks, optical fibers are used to carry optical data signals between optical communications devices connected on opposite ends of optical fibers. In some optical communications networks, a device known as a 2-to-1 optical coupler is used to optically couple signals between an end of an optical fiber and transmit and receive channels of an optical communications device. The 2-to-1 optical coupler is generally an optical splitter comprising a branch-like structure having first, second and third ends. A first branch of the optical coupler extends between the first and second ends of the optical coupler. A second branch of the coupler extends between the first and third ends of the optical coupler.
2-to-1 optical couplers are used in a variety of applications, including, for example, bi-directional communications over POFs. When used for bi-directional communications, the first end of the optical coupler is connected to a first end of a main POF and the second and third ends of the coupler are connected to transmit and receive sides, respectively, of an optical transceiver. In a transmit mode, optical data signals generated on the transmit side of the optical transceiver are passively routed over the 2-to-1 optical coupler from the second end of the coupler to the first end of the coupler. As the optical data signals arrive at the first end of the optical coupler, they are optically coupled into the first end of the main POF. In a receive mode, optical data signals that pass out of the first end of the main POF pass into the first end of the optical coupler and are then passively routed along the second branch of the coupler from the first end of the coupler to the third end of the optical coupler. As the optical data signals pass out of the third end of the coupler, they are received in the receive side of the optical transceiver.
FIG. 1A illustrates a side view of a portion of a typical POF bi-directional optical communications link, which includes a 1.0 millimeter (mm) POF 2 and a 2-to-1 optical coupler 3. The POF 2 functions as the main optical fiber of the link. In one direction, the 2-to-1 optical coupler 3 routes optical signals generated by a transmitter (Tx) onto an end face 2a of the main POF. In the other direction, the 2-to-1 optical coupler 3 routes optical signals passing out of the end face 2a of the main POF 2 onto a photosensor (not shown) of receiver (Rx) 6. The Tx 5 and the Rx 6 are typically parts of an optical transceiver module (not shown). The main POF 2 is referred to above as a 1.0 mm POF due to the fact that the diameter of the core of the POF 2 is 1.0 mm. The 2-to-1 optical coupler 3 has the branch-like splitter configuration described above, with each branch comprising a respective branch POF 3a and 3b. The branch POFs 3a and 3b are typically also 1.0 mm POFs. The end face 2a of the main POF 2 has a cross-sectional area equal to 8/32π, where π=3.14159. Likewise, the end faces 3c and 3d of the branch POFs 3a and 3b, respectively, have cross-sectional areas equal to 8/32π. However, the end faces of the branch POFs 3a and 3b that interface with the end face 2a of the main POF 2 are each reduced in cross-sectional area by approximately 50% to form a coupler end face 3e having a cross-sectional area of 8/32π, which matches the cross-sectional area of the end face 2a of the main POF 2.
FIGS. 1B and 1C illustrate front plan views of the end faces 2a and 3e of the main POF 2 and of the coupler 3, respectively. It can be seen from FIGS. 1B and 1C that the end faces 2a and 3e have equal cross-sectional areas. A variety of techniques may be used to reduce the cross-sectional areas of the end faces of the branch POFs 3a and 3b to form the coupler end face 3e. Polishing and chisel cutting are two well know techniques that are used for this purpose. In addition, in some cases a technique known as metal evaporation is used to form a metal layer 7 between the branch POFs 3a and 3b at the coupler end face 3e to prevent light from being coupled between the branch POFs, i.e., to prevent optical cross-talk. A configuration of the type shown in FIGS. 1A-1C is disclosed in U.S. Pat. No. 7,206,493. Another technique for varying the cross-sectional areas of the end faces of the branch POFs is a hot molding technique that uses a molding tool in combination with heat to provide the coupler end face with a desired non-circular cross-sectional shape. Such a technique is disclosed in U.S. Pat. No. 6,473,555.
The use of the aforementioned techniques of chisel cutting, polishing and hot molding to form the non-circular cross-sectional end faces increases manufacturing costs and limits production throughput. A need exists for a 2-to-1 POF optical coupler in which the non-circular cross-sectional end faces can be produced at relatively low manufacturing costs and with relatively high manufacturing throughput.